1. Field of the Invention
This invention relates to solid-state photodetecting, especially complementary metal oxide semiconductor (CMOS) or active pixel sensor (APS) photodetecting, in which photodiodes are used as the photodetecting elements. More particularly, the invention relates to a photodetector, to an optoelectronic image sensor using said photodetector and to a method for sensing electromagnetic radiation using said photodetector. The invention is useful for all photodetecting applications in which a dynamic range exceeding three orders of magnitude (60 dB) is required, such as security, surveillance, automotive, soldering, etc. It is also useful for photodetecting applications at elevated temperatures (above 80° to 100° C.), in which exponentially increasing dark current severely limits the dynamic range of conventional photodetectors.
2. Description of Related Art
Solid-state image sensors and cameras find more and more applications in areas where the environmental conditions cannot be well controlled. There are two main problems resulting from this fact:    A. Firstly, the large variations of illumination in uncontrolled surroundings call for a photodetector with a dynamic range that is significantly larger than the 50-60 dB of conventional cameras.    B. Secondly, high surrounding temperatures (above 80° to 100° C.) cause a high level of dark current that can severely limit the voltage swing of the photodiodes, leading to an unacceptable loss of contrast.A. Dynamic Range
The state of the art in image sensors with high dynamic range is based on two different approaches:                (i) multiple exposures of the same scene with different exposure times and subsequent combination of the obtained image data into one image offering high dynamic range; or        (ii) non-linear compression of the pixel response, by reducing a pixel's sensitivity for high illumination levels.        
A good overview of the different methods in the first category (i) of multiple-exposure techniques is given by O. Yadid-Pecht (“Wide dynamic range sensors”, Optical Engineering, Vol. 38, pp. 1650-1660, 1999). All methods share the common disadvantage that they have to store data of the different exposures, either in analog or in digital form. Often complete images have to be stored, and in the best of cases, several lines or pixels of an image have to be stored, as disclosed by Seitz et al. in EP-1,003,329 A1. A second disadvantage of the multi-exposure methods is the required computational effort necessary for combining data from the differently exposed images, while avoiding image artifacts that arise from the combination of photodetector signals whose properties are not precisely known. This can be caused by cross-talk, dark current, non-linearities in the response or temperature dependent transistor parameters.
The second category (ii) of methods that obtain a high dynamic range by response compression encompasses logarithmic compression with a metal oxide semiconductor (MOS) field effect transistor (FET), as taught by H. Graf et al. (“Elektronisch Sehen”, Elektronik, Vol. 3, pp. 3-7, 1995). A disadvantage of such a “logarithmic pixel” is the severe reduction of the response speed at low illumination levels. This can be overcome by combining a linear response at low light levels with a logarithmic response at high light levels. This is achieved by adding a reset transistor in parallel to the transistor that performs the logarithmic compression. International publication WO-01/46655 A1 (M. Wäny) teaches how the same functionality can be obtained with a single transistor whose gate is driven with a suitable voltage pattern. In all three cases, transistors are employed as non-linear elements that have some disadvantages in this application. The threshold voltage and, therefore, the on-set of the logarithmic compression depends on temperature and properties of the oxide that might change with time such as mobile oxide charges. Alternatively, the pixel at the same time requires precise timing (for carrying out the reset operation) and precise voltage levels (for carrying out logarithmic compression) which is not easy to realize. A further disadvantage are the variations of the semiconductor process while fabricating the transistors, resulting in nonuniform parameters of the pixels. As a consequence, the response of the different pixels must be adapted to each other, either by digital post-processing, or by increasing the size of the transistors that carry out the logarithmic compression to reduce the variations from pixel to pixel.
B. Dark Current
The second practical problem addressed by the present invention is a photodiode's dark current that doubles for every increase in temperature by about 8° C. If temperatures approach 100° C. or if they are even increased above, as it can occur for example in automotive applications, the dark current can be much larger than the photocurrent. This leads to a situation in which the photosignal is drowned in the temperature-induced offset signal. This problem is conventionally addressed by improving the employed semiconductor process in terms of reduced dark current. However, process changes involve higher costs and technical problems.